Research suggests there are 25,000 fewer addicts in 2009-10 than six years ago when the first survey by experts from the Centre for Drug Misuse Research at the University of Glasgow was carried out.

However, banning the legal high mephedrone in 2009 has had the perverse effect of increasing the danger to drug users, experts say.

The latest estimate of 306,150 opiate and crack cocaine users in 2009-10 is the first to show a significant decrease. Of those, 103,185 were injecting drug users who face the greatest danger, a fall of 12 per cent.

Paul Hayes, chief executive of the National Treatment Agency for Substance Misuse, said: "It looks as if fewer young adults are turning to heroin and crack and fewer users in general are taking part in risky injecting behaviour. This is an encouraging development but we can't be complacent as long as drugs are ruining lives and causing misery to communities."

The findings come after figures published by the Office for National Statistics showed deaths from drug poisoning, involving both legal and illegal drugs, fell 5 per cent in 2010 compared with 2009. Deaths involving cocaine fell 29 per cent to 144. Experts said there had been a significant drop in the number of young adults coming for treatment with drug problems.

"The demand is from older entrenched users who were caught up in the heroin epidemic of the 1980s. They are a more difficult nut to crack", a spokesman for the NTA said.

Gordon Hay, who led the study, said the number of users had been falling consistently for five years but this was the first statistically significant drop.

"It is going down more in younger age groups – both because they are less likely to use heroin and crack than the older age groups and because there are effective ways of dealing with them," he said.

There had been a "massive expansion" of treatment and it was much quicker for addicts to get help than in the past, he said.

Sheila Bird, of the MRC Biostatistics Unit, Cambridge, who is an expert on drug addiction, said previous estimates by the University of Glasgow group had been too low.

"When Gordon Hay said there were 130,000 injecting users, we estimated 200,000. When the answer you get depends on the assumptions you make, we have to be cautious about calling a triumph."

Professor Bird said the sharp fall in deaths involving cocaine suggested users had switched to the legal high mephedrone, which was of higher quality and less likely to be "cut" with other substances. "Cocaine is a lethal drug," she said.

Mephedrone was banned controversially by the Government in March 2009. Prof Bird said: "Banning mephedrone may have had an adverse effect on the public health because when it was around we were spared the cocaine-related deaths.

"That raises the question: is there anything we can do in public health that has the effect we have seen? We need to learn from this."